Bridgeport 2J-2 Head Noise

There is a resonance in the head of my BOSS 6 milling machine that
occurs at medium to high speeds and causes a knocking noise. The
noise appears to come from the spindle pulley area. It happens even
with the gear box in neutral, so I think the gear box and spindle
bearings can be ruled out. The three upper head bearings are new, as
well as the spindle pulley hub and brake bearing cap. Everything
seems tight. Anyone have any ideas as to what the cause might be?

~petev